An assay is an investigative procedure for determining the presence, quantity, activity, and/or other properties or characteristics of components in a sample. Very often, the components of interest within a sample e.g., a nucleic acid, an enzyme, a virus, a bacterium are only minor constituents of the sample and may, therefore, be difficult to detect or quantify.
An example of a biological assay is a polymerase chain reaction (PCR) assay. Certain types of PCR can be quantitative in specific settings. For example, real-time PCR (which generally involves monitoring the progression of amplification using fluorescence probes) may permit quantification of target nucleic acids in a sample, particularly where the target nucleic acids are somewhat abundant.
Droplet digital polymerase chain reaction (ddPCR) is a popular tool for quantitative measurement of absolute DNA concentration. As opposed to the relative measurement obtained from the real-time PCR reactions, ddPCR enables absolute measurement of target deoxyribonucleic acid (DNA), ribonucleic acid (RNA) molecules. Absolute quantification is advantageous in several applications, such as measuring copy number variation, detecting rare sequences and mutations, and analyzing gene expression reaction.